Near infrared spectral studies on interactions of CH 3 groups with halide ions

Transcription

1 Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 105, No. t, February 1993, pp Printed in India. Near infrared spectral studies on interactions of CH 3 groups with halide ions A F JAYARAJ and SURJIT SINGH* Department of Chemistry, Indian Institute of Technology, Madras , India MS received 9 July 1992 Abstract. Near infrared (NIR) spectra in the CH 3 stretching first-overtone region for methyl iodide, acetonitrile, nitromethane and dimethyl sulphoxide are discussed in terms of normal mode and local mode descriptions. NIR spectra in this region for solutions of several alkali metal halides and tetraalkyl ammonium halides in these solvents are also reported. It is found that the relative intensity of the cooperative excitation band with respect to the pure overtone excitation band reduces considerably on increase in the concentration of the electrolytes in solution. The results are discussed in terms of the formation of anion-molecular complexes as well as intermode coupling due to kinetic energy and anharmonicity effects. Keywords. Anion-molecular interactions; overtone spectra; local modes; methyl iodide; acetonitrile; nitromethane; dimethyl sulphoxide. 1. Introduction Molecular interactions through methyl groups of organic molecules have been reported earlier (e.g. Mitchell and Lippert 1978, Sastry and Singh 1986). It is suggested (Mitchell and Lippert 1978) that the effect of axial interactions on the CH 3 group on acetonitrile (AN) may be observed with respect to its.symmetric stretching mode whereas the perpendicular interactions can be studied with respect to the antisymmetric stretching mode. A change in the intensity of the antisymmetric CH 3 stretching fundamental band was observed with variation in solvent and temperature (De Maine 1960; Mahnke 1971; Mitchell and Lippert 1978; E Knoezinger, private communication), and by means of the equilibrium model an association enthalpy of 6 kjmol- 1 was calculated for solutions of acetonitrile in carbon tetrachloride (Mahnke 1971). Studies on the interactions of the CH3 group of AN with halide ions employing IR and Raman spectroscopy have been reported (Roche and Huong 1979; Mitchell and Lippert 1978; Sastry and Singh 1986; Venkata Ramana and Singh 1988, 1989a) in the C-H (D) stretching region. It was observed (E Knoezinger, private communication) that the peak heights of the fundamental bands for the CH 3 antisymmetric and symmetric stretching modes of pure AN are comparable in its IR spectrum, and as the concentration of AN in its solution in carbon tetrachloride decreases, the relative intensity of the antisymmetric band with respect to the symmetric * For correspondence 71

2 72 A F Jayaraj and Surjit Singh band gets reduced. In IR spectra of solutions of alkali metal halides in AN (Roche and Huong 1970) it is observed that both the bands show splittings because of the existence of an equilibrium between the bulk and associated AN, and on further increase in concentration of salts the bulk AN bands almost disappear. The antisymmetric CH a stretching band of associated AN has lower intensity than the symmetric CH3 band, even though the overall intensities of both the bands are found to be much higher for associated AN than those of pure AN. The above observations seem to indicate that the ratio of the intensities of antisymm~tric to symmetric CH3 bands in the fundamental stretching region is less than unity for free AN as well as for AN associated with halide ion, whereas it is close to unity for liquid AN. The CH3 symmetric stretching band of AN (Venkata Ramana and Singh 1988) and CH3 symmetric as well as antisymmetric stretching bands of dimethyl sulphoxide [DMSO] in Raman spectra also show (Venkata Ramana and Singh 1989b) splittings in the presence of halide ions, thus indicating the involvement of the CH 3 group in AN and DMSO on interactions with halide ions. As an extension to our work on the studies of the interaction of CH a groups with halide ions we now report near infrared (NIR) spectral studies of solutions of alkali metal- and tetraalkyl ammonium halides in methyl iodide (MI), acetonitrile (AN), nitromethane (NM) and dimethyl sulphoxide (DMSO). 2. Experimental MI (Sisco), AN (Fluka), NM (Farek) and DMSO (Koch-light) were used after drying over 4/~, molecular sieves. Lithium halides (Fluka-AG), potassium halides (Merck), tetrabutyl ammonium bromide (Sigma), tetrabutyl ammonium iodide (Sigma), tetraethyl ammonium bromide (Sigma) and tetraheptyl ammonium bromide (Fluka AG) were used without further purification. IR spectra were recorded on a Varian 2390 spectrometer with a scan rate of 0-2 nm/s, response time of 10s and spectral band width of 2 nm. 3. Results and discussion In figure 1 are given NIR spectra of MI, AN, NM and DMSO in the first overtone region of the CH 3 stretching mode. For molecules having XY 3 functional groups the three X-Y bonds can be combined to give the symmetry coordinates A t and E corresponding to the three normal modes of vibration of the parent molecule. If the wavefunctions in terms of normal modes consisting of the XY 3 stretching vibrations with A 1 and E symmetries are given in terms of vibrational quantum numbers n, and no as In s, na) then for the first overtone (n s + n a = 2) for XY 3 stretching vibrations, the three wavefunctions can be depicted as 12,0), 10,2) and Jl, 1). The 12,0) wavefunction belongs to symmetry A1 whereas the [0,2) wavefunction, because of degeneracy of the asymmetric mode, splits into two wavefunctions J0,2)A t and 10, 2)E. The combination wavefunction [1, 1) belongs to the E symmetry species. Recently, NIR spectra of molecules with CH 3 groups have been reported in terms of local mode theory (Halonen and Child 1983; Henry et al 1983; Ahmed and Henry 1987, 1988). It is found that, in general, molecules containing CH3 groups tend to

3 Interactions of CH3 groups with halide ions >A >E // \ / // / ul,5 m m < / O5 / / / / /// / v WAVE LENGTH (nm) Figure 1. NIR spectra of methyl iodide (MI), acetonitrile (AN), nitromethane (NM) and dimethyl sulphoxide (DMSO) in first overtone region of the CH 3 stretching mode. have similar band structures in various C-H stretching overtone regions with varying relative intensities of component bands within the region of the specific overtone of interest. Various components are assigned to pure overtones, local mode combinations (or cooperative excitations) and intermode combinations of local mode excitations with skeletal excitations. Definite correlations have been worked out (Sage and Jortner 1981; Child and Halonen 1984; Mills and Robiette 1985) between local mode and normal mode models for observed transitions. Ahmed and Henry (1987, 1988) discussed the assignments for NIR spectra of AN and AN (d3) along with methyl halides in the overtone region and have proposed assignments for various bands on the basis of local mode theory. If the vibrational quantum numbers for three X-Y oscillators are designated by 11, 12 and 13, various wavefunctions Ill, 12,13 ) for the first overtone level (l t = 2) can be designated as 12,0,0), 10,2,0), 10,0,2), II, 1,0), 11,0, 1) and J0, I, 1). The first three wavefunctions with one of the local modes having a vibrational quantum number li = 2 are pure overtone levels and the remaining three with two local modes having one vibrational quantum number each are called cooperative excitation levels. Both these groups of wavefunctions have three equivalent functions which on projection operation give rise to two symmetry-adapted linear combinations (A1 and E) each designated as ]2, 0, 0) A1, 12, 0, 0) E, I1, 1, 0) A t and 11, 1, 0) E, respectively. The two sets of wavefunctions under local and normal mode models with designated symmetries are correlated (Halonen and Child 1983; Child and Halonen 1984) as shown in figure 2. The transition from ground state to high energy state with wavefunctions in terms of local mode for MI and AN are shown in figure 1. The assignments for

4 74 A F Jayaraj and Surjit Singh Fundomental First overtone First overtone Fundomentol t02>e >E r1101> ti110> I 02>A~ 1110>A1L1011> Ill >E..... I 200>E[1020> ~i 1200> I 2C' >A1 I 200>AIL1002 > 101>E - - ] 10>A I >E [ooi> ~010> I 1oo>~,L~oo > Normal mode description Local mode description Figure 2. Normal mode to local mode correlation diagrams of the CHa group. NM and DMSO are not available in the literature and therefore tentative assignments are assumed in the following section by comparison with MI and AN. 3.1 NIR spectra of solutions of halide ions NIR spectra of solutions of alkali metal halides as well as tetraalkyl ammonium salts in MI, AN, NM and DMSO at various concentrations were recorded. A stack diagram showing the changes in band contour of the first overtone of the CH 3 stretching mode of MI in solutions of tetraheptyl ammonium bromide in MI is shown in figure 3. As can be noticed, with an increase in concentration of salt, the relative intensity of the I1, 1,0)A1 band seems to reduce with respect to both 12,0,0)A1 and 12,0,0)E bands. The results of band fit analysis for this contour for various tetraalkyl ammonium salts dissolved in methyl iodide are given in table 1. The relative intensities in terms of 12, 0, 0)/I 1, 1, 0) shown in the last two columns indicate changes in the intensity pattern brought about by the interaction of halide ions with CH3 groups of MI. Since 12,0,0)A1 band is found to be overlapped by 12,0,0)E band in AN and other solvents, further studies on effect of interactions on the relative intensities were confined to I2,0,0)E/]I, 1,0)A1 bands only. In table 2 are shown the results of the analysis of band fitting on C-H first overtone band profiles of various systems of electrolytes and organic solvents exhibiting X-... n3c interactions. In figure 1 the band assignments have been shown for MI and AN. On comparison of band contours of four solvents it is assumed that the corresponding bands for 12, 0, 0) E and I1, 1, 0) A t for NM and DMSO have frequencies , nm and , nm, respectively. On the basis of these assignments the relative intensities of 12,0,0)E and I1, 1,0)A~ bands show similar trends for solutions of electrolytes in NM and DMSO as found for the solutions of the corresponding electrolytes in MI and AN. The two bands for 12,0,0)E, I1, 1,0)A~ in local mode description are correlated to I1, 1 >E and 10, 2)A~ under normal mode description. As reported earlier (Roche and Huong 1970), in complexes of AN with alkali metal

5 Interactions of CH 3 groups with halide ions 75 ~ I O7 I... /-/7 z < 0,< ~05 / / /... 7/ // 03 /I / _. //" z o] j ' a20 WAVE LENGTM (nm) f Figure 3. NIR spectra of solutions of THAB in MI in the first overtone region of CH 3 stretching mode. Concentration of THAB (a) pure, (b) 0-0I M, (c) 0-02 M, (d) 0.03 M, (e) 0-04 M, and (0 saturated. / I/" Table 1. Spectral parameters for CH 3 stretching bands in first-overtone region for solutions of tetraalkyl ammonium salts in methyl iodide. Electrolyte Concentration Frequency (nm) of electrolyte (mol/l) 1200>A~ [200>E II10>A~ II200>A, II200> E Illl0>A~ IIII0>A ~ TBAB TBAI THAB " : ' '2 1694"2 1655' '56 0"35 0' '3 1656" ' ' "55 0" ' THAB- tetrabutyl ammonium bromide; TBAI- tetrabutyl ammonium iodide; THAB- tetraheptyl ammonium bromide. halides the relative intensity of the antisymmetric stretching fundamental to the symmetric stretching fundamental is less than unity whereas for pure AN the ratio is approximately unity. In the present studies, on formation of complexes between organic molecules and halide ions through interaction with CH 3 groups, the 10, 2>At band is found to decrease in intensity on increasing the concentration of salt with respect to the I1, 1 > E band. Since no splitting of the original band is observed on dissolving halide ions in organic solvents, it can be surmised that CH 3 bands due to both the free as well as associated organic solvent molecules appear at similar positions in the NIR spectra. The change in the relative intensity therefore can be qualitatively

6 76 A F Jayaraj and Surjit Singh explained on the basis of change on complexation in the extinction coefficients of CH3-stretching NIR bands. A variation in the extent of change in the relative intensities of the two CH 3 bands for systems with similar compositions of salt solutions can be explained on the basis of the varying extinction coefficients of ion-molecular complexes with the varying nature of halide ions as well as solvent molecules. Alternatively, if one assumes the extinction coefficients of anion-solvent complexes to be similar, variation in the relative intensities can be employed to quantitatively predict the strength and extent of molecular interactions. Since we do not know the extinction coefficients of anion-molecular complexes, we have not made any attempt to derive quantitative trends regarding the strength of anion-molecular complexes Table 2. Spectral parameters for CH s stretching bands in first overtone region for solutions of metal halides and tetraalkyl ammonium halides in AN, NM and DMSO. Concentration Frequency (nm) of electrolyte Electrolyte (mol/1) 1200> E I 110> A I 11200> E /lllo>a, Acetonitrile 1714-t TBAB "6 1680" " TBAI " ' " THAB 0" ' " KI ' " LiBr 0'25! !-00 LiI 0' "19 Nitromethane ' TBAB 0" " " TBAI 0" '01 0" " DMSO 1687' TBAB 0" '8 0"42 0" '7 1655" TBAI 0' "0 1657' "9 1657" THAB 0" '7 1657' "3 1656' TEAB 0" "8 0" LiBr 0" '2 1659" " ' KI 0' "9 1658"8 0'41 1' "4 1659"3 0"46 *DMSO - dimethylsulphoxide; TBAB - tetramethyl ammonium bromide; TBAI - tetrabutyl ammonium iodide; THAB - tetraheptyl ammonium bromide; TEAB - tetraethyl ammonium bromide

7 Interactions of CH 3 #roups with halide ions 77 using the present data, even though some familiar trends in this direction are quite evident from the values given in table 2. Burberry and co-workers (Burberry and Albrecht 1979; Burberry et al 1979) have reported the intensities of CH-stretching overtone spectra in terms of the local modes for a variety of hydrocarbons. It was found that for a given overtone all CH local modes contribute equally to the intensity. These results provide compelling evidence for the validity of local mode descriptions of overtone spectra. In addition to the pure overtone transition, combination bands corresponding to one-photon cooperative excitation of two bond modes are also observed which provide experimental evidence for the breakdown of the additivity concept in the excitation of CH bonds. An understanding of the nature of the cooperative excitation gives interesting information regarding intramolecular coupling between bond modes. It is observed, in general, that the relative intensity of cooperative excitation with respect to pure overtone transition decreases considerably with an increase in the order of overtone excitation along with a decrease in energy separation between the two bands. For CH oscillators of dihalomethanes (Sage and Jortner 1981), the relative intensity of cooperative excitation with respect to the conventional one mode-overtone excitation is given by r = I /I -~ (4< W>/AE) 2 O11; 2 00"r O"*U where (W) represents, (i) the perturbation-containing kinetic and potential energy terms which couples the bond modes, as well as, (ii) the kinetic and potential energy coupling for bond and skeletal modes. The term AE=2VI V2x, where x is the anharmonicity and i"1, V2 are the vibrational quantum numbers for local bond modes. Thus the relative intensity of cooperative excitation with respect to pure local mode excitation is a measure of intermode coupling due to kinetic energy and anharmonicity effects and a decrease in the relative intensity with increasing order of overtone excitation indicates a decrease in intermode coupling. A similar reasoning may also hold good for the variation in the intensity of the cooperative excitation with respect to pure overtone excitation on increasing the concentration of electrolytes in the solutions. This is being looked into quantitatively by analysing the mathematical terms involved in the above expression and the results of stich a study will be reported in due course. 4. Conclusions The CH 3 first-overtone band contour is found to change on increase in concentration of halide ions for their solutions in all the four solvents considered. The effect is larger for solutions of tetraalkyl ammonium salts than for metal halides. The variation in relative intensity of the pure overtone excitation band with respect to the cooperative excitation band shows familiar trends representative of the nature of the halide ion, the tetraakyl ammonium groups as well as the solvents used. However, quantitative conclusions cannot be drawn because of the lack of information on the extinction coefficients of the anion-molecular complexes. The local mode description for explaining the changes in the relative intensities of the two bands under consideration, in terms of intermode coupling due to kinetic energy and anharmonicity effects, needs further exploration for better understanding of the intermode coupling in electrolyte solution.

8 78 A F Jayaraj and Surjit Singh Acknowledgement One of us (JAF) is grateful to the Indian Institute of Technology, Madras, for the award of a fellowship. References Ahmed M K and Henry B R 1987 J. Chem. Phys Ahmed M K and Henry B R 1988 Can. J. Chem Burberry M S and Albrecht A C 1979a J. Chem. Phys Burberry M S, Morell J A, Albrecht A C and Swofford R L 1979b J. Chem. Phys Child M S and Halonen L 1984 Adv. Chem. Phys De Maine P A D 1960 d. Mol. Spectrosc Halonen L and Child M S 1983 J. Chem. Phys Henry B R, Tarr A W, Mortenson O S, Murphy W F and Compton D A C 1983 J. Chem. Phys Mahnke H 1971 Diplomarbeit (Berlin, Germany: Technical University) Mills I M and Robiette A G 1985 Mol. Phys Mitchell H and Lippert E 1978 Organic liquids: structure, dynamics and chemical properties (eds) A D Buckingham, E Lippert and S Bratos (New York: John Wiley) Roche J P and Huong P V 1970 J. Chem. Phys Sage M L and Jortner J 1981 Adv. Chem. Phys Sastry M I S and Singh S 1986 Curr. Sci Venkata Ramana K and Singh S 1988 Spectrochem. Acta. A Venkata Ramana K and Singh S 1989a J. Mol. Struct Venkata Ramana K and Singh S 1989b J. Raman Spectrosc